Virtually all multiple-access communication systems today employ some form of forward error correction. A multiple-access communication system is a system in which multiple users can simultaneously communicate over the same channel. A code division multiple access (CDMA) system is a multiuser system in which signals of different users are spread over a wide frequency band using different spreading codes. The despreader in such a system uses the spreading code of each individual user to despread that user's signal and obtain the originally transmitted data. A decoder, such as a Viterbi or Turbo decoder, may be used to correct bit errors in the received signal for each user in systems which employ convolutional encoders at the transmitter.
The capacity of CDMA systems in mobile environments is mainly limited by multiple-access interference (MAI) and fading. Multi-user detectors and error-control coding are two primary means to improve performance of CDMA in wireless systems. While the channel coding is specified in any communication standard, multi-user detection is a differentiating receiver feature for manufacturers. Recently, parallel interference cancellation (PIC) has received much interest for base station receivers as it is more suited to the uplink due to its asynchronous nature which, together with long spreading codes, makes interference suppression using other multi-user detectors difficult. Moreover, PIC is well-suited for iterative demodulation and decoding. In such an arrangement, single-user Viterbi or Turbo detectors are typically used for each user.
In dealing with PIC and decoding, the residual MAI (RMAI) has predominantly been modeled as Gaussian. However, the structured RMAI after PIC has statistics that are quite different from Gaussian. This casual application of the so-called Gaussian assumption has resulted in branch or transition metrics for the Viterbi decoder and log-likelihood ratios (LLRs) for the Turbo decoder that offer weaker coding gains than possible. Accordingly, there exists a long-felt need for improved methods and systems for demodulating a multiuser signal using channel decoders for a multiple-access communication system.